Slurry polymerization for preparing ethylene polymers is a known technique in which a non-polymerizable hydrocarbon diluent is used as reaction medium. This kind of polymerization is generally carried out in a turbulent flow reactor, such as a continuous pipe reactor in the form of a loop, or in continuous stirred tank reactors (CSTR). The so-called loop reactor is well known and is described in the Encyclopedia of Chemical Technology, 3rd edition, vol. 16 page 390. In the loop reactor and the CSTR as well LLDPE and HDPE resins can be produced.
The MWD is a particularly important characteristic for ethylene (co) polymers, in that it affects both the rheological behavior and therefore the processability of the melt, and the final mechanical properties. Polyolefins having a broad MWD, particularly coupled with relatively high average molecular weights, are preferred in high speed extrusion processing (films and pipe manufacturing) conditions in which a narrow MWD could cause melt fracture.
Polyethylene to be used in blow molding applications and in particular for large blow molding manufacturing, may have different requirements I terms of MWD. In fact, the broadening the MWD causes the melt strength of the polymer to be higher which, as a consequence, increases the difficulty in cutting off the pinch-off weld (excess of the resin outside the mold during extrusion blow molding) of the final finished article.
Polyethylene having broad MWD can be prepared by multi-step process based on the production of different molecular weight polymer fractions in each single stage, sequentially forming macromolecules with different length on the catalyst particles.
To this end, it can be used a double loop reactor where the two reactors are connected in series in which a high molecular weight fraction may be produced in the first loop reactor and a low molecular weight fraction may be produced in the second loop reactor. In this way, a bimodal polymer or a polymer having a broad molecular weight distribution is made.
A multimodal polyethylene can also be prepared via a multistage reaction sequence comprising successive slurry polymerization stages carried out under predetermined and different reaction conditions in continuously stirred tank reactors so as to obtain polyethylene fractions having different average molecular weight. In this case, monomers and a molar mass regulator, preferably hydrogen, are firstly polymerized in a first reactor under a first set of reaction conditions in the presence of the suspension medium and a suitable catalyst, preferably a Ziegler catalyst, then transferred to a second reactor and further polymerized under a second set of reaction conditions, and, if the polyethylene to be prepared is for example trimodal, further transferred to a third reactor and further polymerized under a third set of reaction conditions, with the first reaction conditions differing from the second and third reaction conditions so as to obtain three polyethylene fractions having different average molecular weight. This difference in molecular weight in the different ethylene polymer fractions is normally evaluated through the determination of the flowability index.
A process of this type was disclosed in WO2010/097305 which relates to a process for preparing polyethylene for pipe application endowed with a broad molecular weight distribution (melt flow ratio F/P, which is the ratio among the melt index value measured according to ASTM 1238 condition “F” and the melt index value measured according to ASTM 1238 condition “P” of higher than 25) carried out in a multistage process with a solid catalyst component comprising Ti, Mg, halogen, and having a specific porosity.
While the process seems to work well under the conditions employed for the preparation of very broad MWD, it suffers from bad operability (pluggage of outer reactor cooled) and low activities when the polymerization conditions are set for the preparation of narrower molecular weight distribution polyethylene.
The applicant has now found that process operability and activity can be improved by a specific modification of the catalyst component.